The present application relates to improvements in processes and systems for the treatment of organic wastes, and in particular, to improvements in processes and systems to reduce net biosolids production in system that utilize biological treatment processes.
Numerous processes have been developed to produce environmentally acceptable effluents through the biological treatment of organic wastes. In biological waste treatment processes, wastewater is contacted with active biosolids (i.e., biosolids containing live micro-organisms) to convert the organic components of the waste to a form that can be separated from the aqueous effluent. However, these processes, usually produce substantial quantities of sludge, consisting primarily of microorganisms and other biosolids in water, which must then be subjected to further processing and disposal, by methods such as incineration, disposal on land or in the ocean, and other chemical, biological or mechanical processes.
Sludge digestion, comprising further biological treatment of the biosolids, has been widely used to reduce the volume of sludge prior to disposal. The main drawback to this approach is that it requires long detention times and correspondingly large treatment systems. Many processes have been proposed to improve the efficiency of sludge digestion. For example, in U.S. Pat. Nos. 3,547,814 and 3,670,887, gross solids are first removed from the sewage by screening and the remaining waste contacted with an oxygen-containing gas and activated sludge.
Another technique for treating sludge involves extended aeration, which increases the degree of auto-oxidation of the solids, with a net reduction of biosolids production. Unfortunately, the rate of oxidation is generally too low to have a significant effect on net sludge production. Moreover, the large plant size and high operating costs of these systems often makes extended aeration impractical and uneconomic.
U.S. Pat. No. 4,246,099 discloses a combination of aerobic/anaerobic processes to reduce and stabilize biosolids produced by an activated sludge process. In one process, sludge from a municipal treatment plant is initially contacted with an oxygen-containing gas under aerobic treatment conditions to partially reduce the biodegradable volatile suspended solids and then anaerobically digested to partially stabilize the sludge. Thermal aerobic digestion, referred to in the reference as autothermal aerobic digestion (ATAD), is utilized, wherein the digester was operated at elevated temperatures, e.g., from about 45xc2x0 C. to 75xc2x0 C., or in the thermophilic range. The anaerobic step is used to solubilize organic compounds biologically, and stabilize the sludge solids. The combination of processes purportedly reduces biosolids in the sludge to less than 40% of the biodegradable volatile suspended solids introduced to the digestion zone.
U.S. Pat. No. 4,026,793 discloses an aerobic digestion process for reducing the solids content in a biodegradable organic sludge by carrying out the digestion in a vessel maintained at a temperature within the range of 38xc2x0 C. to 46xc2x0 C.
European Patent Application EP 0936190 discloses a process wherein solids separated from a biological treatment step are subjected to wet air oxidation to improve the biodegradability of the biosolids or solid waste and return it to the biological reactor. The chemical treatment step entails the use of an oxidant gas such as air or pure oxygen and treatment at temperatures in the range of 80xc2x0 to 200xc2x0 C. and pressures of 1 to 40 atmospheres. The pH of the reaction may range from 1 to 11 and reaction times may range from 10 to 180 minutes. The oxidants are added to the waste mixture in a selected stoichiometric ratio based on the estimated COD of the waste, which generally results in the use of amounts of chemical reagents far greater than are actually needed. Wet air oxidation (WAO) schemes are also expensive and difficult to control and operate. For example, as the waste composition changes, it becomes necessary to increase the reaction temperature and pressure to successfully treat more recalcitrant wastes. Accordingly, plant equipment must be designed for the highest conditions of pressure and temperature that are anticipated. The WAO process design also does not define or recognize all of sludge recycled to the bioreactor in controlling solids production.
U.S. Pat. No. 5,948,275 discloses the use of WAO to treat biosolids with recycle of the biosolids to the WAO process. A primary objective of such treatment is to convert biosolids to gases by the WAO process. The possibility of extracting a partially oxidized effluent for biological treatment is also disclosed.
U.S. Pat. Nos. 5,965,096 and 5,972,226 disclose the use of multiple oxidants to destroy a mixture of biosolids and organics in a WAO process. Oxidants are added in stoichiometric ratios to the COD of the mixture.
U.S. Pat. No. 6,126,838 employs Fenton""s reagent to catalyze oxidization of a high-strength organic waste stream with continuous electrolytic regeneration of the catalyst outside of the oxidation step.
In previous patents, this applicant has disclosed improved sludge digestion processes that employ chemical hydrolysis or chemical oxidation steps to improve the biodegradability of cellular materials and unconverted organic compounds. U.S. Pat. No. 4,915,840 to Rozich discloses an improvement for sludge reduction in an aerobic process for municipal waste. Sludge reduction is controlled by contacting a portion of the biosolids and nonconverted organics with acid or base to hydrolyze macromolecular organic substances and dissolve inorganic substances in the waste stream. This hydrolytic assist modifies the macromolecular components of the cell structures and renders them essentially soluble, thereby enhancing the ability of the biologically active organisms to effect thermophilic decay within the bioreactor.
U.S. Pat. No. 5,141,646 to Rozich discloses a process wherein sludge is charged directly to an ATAD to provide immediate digestion. During quiescent periods, a portion of settled biosolids is then removed from the ATAD reactor and charged to a chemical hydrolysis unit for treatment with a strong acid or base solution. The hydrolyzed stream is mixed with the incoming sludge, which is then fed directly to the ATAD.
U.S. Pat. No. 5,492,624 also to Rozich discloses a process for the treatment of organic waste wherein a chemical oxidation step is employed in place of the chemical hydrolysis step employed in the ""840 or ""646 patents. Chemical oxidation substantially reduces the chemical oxygen demand (COD) of the organic material, improving the efficiency and lowering the overall oxidation demand in the ATAD reactor. The process does not generate the copious amounts of dissolved solids produced by the addition of the acids or bases needed to promote chemical hydrolysis. Dissolved solids can adversely affect the biological treatment step and other downstream treatment processes that may be present. Another advantage of this process is that the solubilization effected by the elevated temperatures in the ATAD takes place before the chemical oxidation step, thus avoiding the redundancies or inefficiencies present in the prior art. The chemical oxidation step is often carried out at temperatures above the atmospheric boiling point of the waste to oxidize recalcitrant compounds. The required temperatures can swing across a broad range as the composition of the influent changes. In such cases, the chemical oxidation equipment is designed for the highest temperatures and pressures that are expected to occur during treatment.
As can be seen from the review of the art pertaining to biological treatment processes, many methods have been proposed in an effort to reduce or minimize sludge production and to stabilize excess sludge produced by aerobic processes. Most of these processes, become quite complex, and, as a result, the operating and capital costs become quite high. Moreover, it is extremely difficult to modify most biological treatment processes to economically achieve substantial sludge reduction, based on original organic input, let alone eliminate sludge production, i.e., to achieve xe2x80x9ca zero sludge system.xe2x80x9d Indeed, the latter goal is one often sought but seldom achieved unless expensive intervening physical separation processes such as dewatering and subsequent incineration are employed.
Processes and systems for reducing sludge produced during biological treatment processes are disclosed. One aspect of the present invention relates to methods for biological treatment of organic waste in which, after subjecting the organic waste to biological digestion, at least a portion of the mixture of biosolids and unconverted organic material is transferred to a chemical treatment unit where it is contacted with at least one oxidizing agent in the chemical treatment unit at a high oxidation/reduction potential (ORP) that is controlled by monitoring ORP of the biosolids/organics mixture and adding oxidizing agents to the mixture as needed to maintain ORP at the selected high level. By maintaining the ORP of the biosolids/organics mixture in the chemical treatment unit at sufficiently high levels, i.e., 0 mV and greater, a highly conditioned effluent can be produced thereby resulting in substantial improvements in biodegradability when the conditioned effluent subsequently is subjected to further biological digestion.
Although not limited to any one theory, the use of ORP to guide the oxidation process is believed to more efficiently control effluent quality independent of the specific oxidizing agents employed because ORP is more directly related to the activity of the oxidizing agents in the mixture. The adjustment of the chemical oxidizing agents based on ORP is also believed to eliminate many of the problems that arise when oxidizing agents are added according to stoichiometric estimates, including, for example, eliminating the need to continually adjust the reaction temperature and pressure to accommodate changes in influent composition, eliminating the risks of underestimating the extent of waste oxidation, and reducing chemical wastage.
In accordance with this aspect of the present invention, a process is provided for the biological treatment of organic waste, comprising:
(a) feeding organic waste to a biological reactor and subjecting the organic waste to biological digestion so as to convert at least a portion of the organic waste to a clear decant and a mixture of biosolids and unconverted organic material;
(b) contacting a least a portion of the mixture of biosolids and unconverted organic material with at least one oxidizing agent in a chemical treatment unit;
(c) monitoring the oxidation-reduction potential (ORP) of the mixture of biosolids and unconverted organic in the chemical treatment unit and adjusting the concentration of the oxidizing agent to maintain the ORP of the mixture at greater than 0 mV so as to convert the biosolids and unconverted organic material to a conditioned effluent; and
(d) returning the conditioned effluent to the biological reactor for further treatment.
In one embodiment of this aspect of the invention, the ORP level of the biosolids/organics mixture is monitored, and a single chemical oxidant, such as oxygen gas, is added to the waste mixture to maintain the ORP at the desired level.
In another embodiment, the ORP of the biosolids/organics mixture is monitored and combinations of chemical oxidants, such as oxygen, permanganate, peroxide, etc., and chemical catalysts, such as ferrous sulfate, peroxide, acid or base, etc., are added to maintain the ORP at the desired level.
In another embodiment of the invention, the ORP of the biosolids/organics mixture is monitored and electrolytic decomposition (electrolysis) of the aqueous mixture is used to generate hydroxy radicals to maintain the ORP at the desired level.
In another embodiment, ORP of the biosolids/organics mixture is adjusted in sequential steps to promote more efficient sludge reduction. The biosolids/organics mixture is contacted with a first oxidizing agent to adjust the ORP of the mixture to about 0 mV or greater, and then the ORP of the biosolids/organics mixture is contacted with a second oxidizing agent to adjust the ORP of the mixture to about +200 mV or greater while the mixture is reacted.
In yet another embodiment, the ORP of the biosolids/organics mixture is adjusted to about +500 mV or higher by the addition of oxidants according to any of the processes disclosed herein and refluxed at pressures near one atmosphere.
The present invention also relates to the integration of various chemical and physical treatment processes into biological treatment systems to achieve substantial reduction in net sludge production approaching zero net sludge production. In an embodiment of this aspect of the present invention, various methods for maintaining the system net growth rate at less than about 0.05 dayxe2x88x921 or increasing respiration rate of the active organisms are integrated with ORP-based control of the chemical oxidation. By maintaining the system net growth rate at less than about 0.05 dayxe2x88x921, biosolid production is minimized in the biological system, thereby minimizing the recirculation of carbon through the system.
Various methods for maintaining the low system net growth rate are disclosed, including separating a portion of the biosolids/organics mixture produced in the biological reactor and recycling it to the biological reactor to control the system net growth rate. Other methods for controlling system net growth rate may also be used, including addition of mature cells to the bioreactor and addition of enzymes to the bioreactor influent to increase biosolids respiration rates and manipulate cell yields.
So that the manner in which the above-recited aspects of the invention are attained can be understood in detail, more particular descriptions of the invention are made by certain embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in reference to the preferred embodiments, it will be appreciated that the preferred embodiments are not intended to limit the invention to the embodiments and that various substitutions and modifications may be made to the invention disclosed without departing from the spirit and scope of the invention.